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MySQL Tutorial MySQL Tutorial Document Transcript

  • MySQL Tutorial
  • MySQL Tutorial Abstract This is the MySQL Tutorial from the MySQL 5.1 Reference Manual. Document generated on: 2010-04-12 (revision: 19969) Copyright © 2008, 2010, Oracle and/or its affiliates. All rights reserved. This software and related documentation are provided under a license agreement containing restrictions on use and disclosure and are protected by intel- lectual property laws. Except as expressly permitted in your license agreement or allowed by law, you may not use, copy, reproduce, translate, broadcast, modify, license, transmit, distribute, exhibit, perform, publish, or display any part, in any form, or by any means. Reverse engineering, disassembly, or decompilation of this software, unless required by law for interoperability, is prohibited. The information contained herein is subject to change without notice and is not warranted to be error-free. If you find any errors, please report them to us in writing. If this software or related documentation is delivered to the U.S. Government or anyone licensing it on behalf of the U.S. Government, the following no- tice is applicable: U.S. GOVERNMENT RIGHTS Programs, software, databases, and related documentation and technical data delivered to U.S. Government customers are "commercial computer software" or "commercial technical data" pursuant to the applicable Federal Acquisition Regulation and agency-specific sup- plemental regulations. As such, the use, duplication, disclosure, modification, and adaptation shall be subject to the restrictions and license terms set forth in the applicable Government contract, and, to the extent applicable by the terms of the Government contract, the additional rights set forth in FAR 52.227-19, Commercial Computer Software License (December 2007). Oracle USA, Inc., 500 Oracle Parkway, Redwood City, CA 94065. This software is developed for general use in a variety of information management applications. It is not developed or intended for use in any inherently dangerous applications, including applications which may create a risk of personal injury. If you use this software in dangerous applications, then you shall be responsible to take all appropriate fail-safe, backup, redundancy, and other measures to ensure the safe use of this software. Oracle Corporation and its affiliates disclaim any liability for any damages caused by use of this software in dangerous applications. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. MySQL is a trademark of Oracle Corporation and/or its affiliates, and shall not be used without Oracle's express written authorization. Other names may be trademarks of their respective owners. This software and documentation may provide access to or information on content, products, and services from third parties. 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This document is not part of your license agreement nor can it be incorporated into any contractual agreement with Oracle or its subsidiaries or affiliates. This documentation is NOT distributed under a GPL license. Use of this documentation is subject to the following terms: You may create a printed copy of this documentation solely for your own personal use. Conversion to other formats is allowed as long as the actual con- tent is not altered or edited in any way. You shall not publish or distribute this documentation in any form or on any media, except if you distribute the documentation in a manner similar to how Oracle disseminates it (that is, electronically for download on a Web site with the software) or on a CD-ROM or similar medium, provided however that the documentation is disseminated together with the software on the same medium. 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For additional documentation on MySQL products, including translations of the documentation into other languages, and downloadable versions in vari- ety of formats, including HTML and PDF formats, see the MySQL Documentation Library.
  • Tutorial This chapter provides a tutorial introduction to MySQL by showing how to use the mysql client program to create and use a simple database. mysql (sometimes referred to as the “terminal monitor” or just “monitor”) is an interactive program that allows you to con- nect to a MySQL server, run queries, and view the results. mysql may also be used in batch mode: you place your queries in a file be- forehand, then tell mysql to execute the contents of the file. Both ways of using mysql are covered here. To see a list of options provided by mysql, invoke it with the --help option: shell> mysql --help This chapter assumes that mysql is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you need to consult the relevant portions of this manual, such as MySQL Server Administration.) This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an existing database, you may want to skip over the sections that describe how to create the database and the tables it contains. Because this chapter is tutorial in nature, many details are necessarily omitted. Consult the relevant sections of the manual for more in- formation on the topics covered here. iv
  • Chapter 1. Connecting to and Disconnecting from the Server To connect to the server, you will usually need to provide a MySQL user name when you invoke mysql and, most likely, a password. If the server runs on a machine other than the one where you log in, you will also need to specify a host name. Contact your administrat- or to find out what connection parameters you should use to connect (that is, what host, user name, and password to use). Once you know the proper parameters, you should be able to connect like this: shell> mysql -h host -u user -p Enter password: ******** host and user represent the host name where your MySQL server is running and the user name of your MySQL account. Substitute appropriate values for your setup. The ******** represents your password; enter it when mysql displays the Enter password: prompt. If that works, you should see some introductory information followed by a mysql> prompt: shell> mysql -h host -u user -p Enter password: ******** Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 25338 to server version: 5.1.47-standard Type 'help;' or 'h' for help. Type 'c' to clear the buffer. mysql> The mysql> prompt tells you that mysql is ready for you to enter commands. If you are logging in on the same machine that MySQL is running on, you can omit the host, and simply use the following: shell> mysql -u user -p If, when you attempt to log in, you get an error message such as ERROR 2002 (HY000): CAN'T CONNECT TO LOCAL MYSQL SERVER THROUGH SOCKET '/TMP/MYSQL.SOCK' (2), it means that the MySQL server daemon (Unix) or service (Windows) is not running. Consult the administrator or see the section of Installing and Upgrading MySQL that is appropriate to your operating system. For help with other problems often encountered when trying to log in, see Common Errors When Using MySQL Programs. Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking mysql without any options: shell> mysql After you have connected successfully, you can disconnect any time by typing QUIT (or q) at the mysql> prompt: mysql> QUIT Bye On Unix, you can also disconnect by pressing Control-D. Most examples in the following sections assume that you are connected to the server. They indicate this by the mysql> prompt. 1
  • Chapter 2. Entering Queries Make sure that you are connected to the server, as discussed in the previous section. Doing so does not in itself select any database to work with, but that is okay. At this point, it is more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how mysql works. Here is a simple command that asks the server to tell you its version number and the current date. Type it in as shown here following the mysql> prompt and press Enter: mysql> SELECT VERSION(), CURRENT_DATE; +-----------------+--------------+ | VERSION() | CURRENT_DATE | +-----------------+--------------+ | 5.1.2-alpha-log | 2005-10-11 | +-----------------+--------------+ 1 row in set (0.01 sec) mysql> This query illustrates several things about mysql: • A command normally consists of an SQL statement followed by a semicolon. (There are some exceptions where a semicolon may be omitted. QUIT, mentioned earlier, is one of them. We'll get to others later.) • When you issue a command, mysql sends it to the server for execution and displays the results, then prints another mysql> prompt to indicate that it is ready for another command. • mysql displays query output in tabular form (rows and columns). The first row contains labels for the columns. The rows following are the query results. Normally, column labels are the names of the columns you fetch from database tables. If you're retrieving the value of an expression rather than a table column (as in the example just shown), mysql labels the column using the expression it- self. • mysql shows how many rows were returned and how long the query took to execute, which gives you a rough idea of server per- formance. These values are imprecise because they represent wall clock time (not CPU or machine time), and because they are af- fected by factors such as server load and network latency. (For brevity, the “rows in set” line is sometimes not shown in the remain- ing examples in this chapter.) Keywords may be entered in any lettercase. The following queries are equivalent: mysql> SELECT VERSION(), CURRENT_DATE; mysql> select version(), current_date; mysql> SeLeCt vErSiOn(), current_DATE; Here is another query. It demonstrates that you can use mysql as a simple calculator: mysql> SELECT SIN(PI()/4), (4+1)*5; +------------------+---------+ | SIN(PI()/4) | (4+1)*5 | +------------------+---------+ | 0.70710678118655 | 25 | +------------------+---------+ 1 row in set (0.02 sec) The queries shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon: mysql> SELECT VERSION(); SELECT NOW(); +-----------------+ | VERSION() | +-----------------+ | 5.1.2-alpha-log | +-----------------+ 1 row in set (0.00 sec) +---------------------+ | NOW() | +---------------------+ | 2005-10-11 15:15:00 | +---------------------+ 1 row in set (0.00 sec) 2
  • Entering Queries A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. mysql determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, mysql accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.) Here is a simple multiple-line statement: mysql> SELECT -> USER() -> , -> CURRENT_DATE; +---------------+--------------+ | USER() | CURRENT_DATE | +---------------+--------------+ | jon@localhost | 2005-10-11 | +---------------+--------------+ In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how mysql indicates that it has not yet seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you can always be aware of what mysql is waiting for. If you decide you do not want to execute a command that you are in the process of entering, cancel it by typing c: mysql> SELECT -> USER() -> c mysql> Here, too, notice the prompt. It switches back to mysql> after you type c, providing feedback to indicate that mysql is ready for a new command. The following table shows each of the prompts you may see and summarizes what they mean about the state that mysql is in. Prompt Meaning mysql> Ready for new command. -> Waiting for next line of multiple-line command. '> Waiting for next line, waiting for completion of a string that began with a single quote (“'”). "> Waiting for next line, waiting for completion of a string that began with a double quote (“"”). `> Waiting for next line, waiting for completion of an identifier that began with a backtick (“`”). /*> Waiting for next line, waiting for completion of a comment that began with /*. Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, mysql waits for more input: mysql> SELECT USER() -> If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely mysql is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and mysql executes it: mysql> SELECT USER() -> ; +---------------+ | USER() | +---------------+ | jon@localhost | +---------------+ The '> and "> prompts occur during string collection (another way of saying that MySQL is waiting for completion of a string). In MySQL, you can write strings surrounded by either “'” or “"” characters (for example, 'hello' or "goodbye"), and mysql lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you have entered a line containing a string that begins with a “'” or “"” quote character, but have not yet entered the matching quote that terminates the string. This often indicates that you have inadvertently left out a quote character. For example: 3
  • Entering Queries mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30; '> If you enter this SELECT statement, then press Enter and wait for the result, nothing happens. Instead of wondering why this query takes so long, notice the clue provided by the '> prompt. It tells you that mysql expects to see the rest of an unterminated string. (Do you see the error in the statement? The string 'Smith is missing the second single quote mark.) At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type c in this case, because mysql interprets it as part of the string that it is collecting. Instead, enter the closing quote character (so mysql knows you've finished the string), then type c: mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30; '> 'c mysql> The prompt changes back to mysql>, indicating that mysql is ready for a new command. The `> prompt is similar to the '> and "> prompts, but indicates that you have begun but not completed a backtick-quoted identifier. It is important to know what the '>, ">, and `> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type appear to be ignored by mysql — including a line containing QUIT. This can be quite confusing, especially if you do not know that you need to supply the terminating quote before you can cancel the current command. 4
  • Chapter 3. Creating and Using a Database Once you know how to enter commands, you are ready to access a database. Suppose that you have several pets in your home (your menagerie) and you would like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer dif- ferent sorts of questions about your animals by retrieving data from the tables. This section shows you how to perform the following op- erations: • Create a database • Create a table • Load data into the table • Retrieve data from the table in various ways • Use multiple tables The menagerie database is simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of data- base might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL Web site. It is available in both compressed tar file and Zip formats at http://dev.mysql.com/doc/. Use the SHOW statement to find out what databases currently exist on the server: mysql> SHOW DATABASES; +----------+ | Database | +----------+ | mysql | | test | | tmp | +----------+ The mysql database describes user access privileges. The test database often is available as a workspace for users to try things out. The list of databases displayed by the statement may be different on your machine; SHOW DATABASES does not show databases that you have no privileges for if you do not have the SHOW DATABASES privilege. See SHOW DATABASES Syntax. If the test database exists, try to access it: mysql> USE test Database changed USE, like QUIT, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The USE statement is special in another way, too: it must be given on a single line. You can use the test database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose that you want to call yours menagerie. The administrator needs to execute a command like this: mysql> GRANT ALL ON menagerie.* TO 'your_mysql_name'@'your_client_host'; where your_mysql_name is the MySQL user name assigned to you and your_client_host is the host from which you connect to the server. 3.1. Creating and Selecting a Database If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to cre- ate it yourself: mysql> CREATE DATABASE menagerie; 5
  • Creating and Using a Database Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as menagerie, not as Menagerie, MENAGERIE, or some other variant. This is also true for table names. (Under Windows, this restriction does not ap- ply, although you must refer to databases and tables using the same lettercase throughout a given query. However, for a variety of reas- ons, the recommended best practice is always to use the same lettercase that was used when the database was created.) Note If you get an error such as ERROR 1044 (42000): ACCESS DENIED FOR USER 'MONTY'@'LOCALHOST' TO DATA- BASE 'MENAGERIE' when attempting to create a database, this means that your user account does not have the necessary privileges to do so. Discuss this with the administrator or see The MySQL Access Privilege System. Creating a database does not select it for use; you must do that explicitly. To make menagerie the current database, use this com- mand: mysql> USE menagerie Database changed Your database needs to be created only once, but you must select it for use each time you begin a mysql session. You can do this by is- suing a USE statement as shown in the example. Alternatively, you can select the database on the command line when you invoke mysql. Just specify its name after any connection parameters that you might need to provide. For example: shell> mysql -h host -u user -p menagerie Enter password: ******** Important menagerie in the command just shown is not your password. If you want to supply your password on the command line after the -p option, you must do so with no intervening space (for example, as -pmypassword, not as -p mypass- word). However, putting your password on the command line is not recommended, because doing so exposes it to snoop- ing by other users logged in on your machine. Note You can see at any time which database is currently selected using SELECT DATABASE(). 3.2. Creating a Table Creating the database is the easy part, but at this point it is empty, as SHOW TABLES tells you: mysql> SHOW TABLES; Empty set (0.00 sec) The harder part is deciding what the structure of your database should be: what tables you need and what columns should be in each of them. You want a table that contains a record for each of your pets. This can be called the pet table, and it should contain, as a bare minim- um, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex. How about age? That might be of interest, but it is not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it is better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too: • You can use the database for tasks such as generating reminders for upcoming pet birthdays. (If you think this type of query is somewhat silly, note that it is the same question you might ask in the context of a business database to identify clients to whom you need to send out birthday greetings in the current week or month, for that computer-assisted personal touch.) • You can calculate age in relation to dates other than the current date. For example, if you store death date in the database, you can easily calculate how old a pet was when it died. 6
  • Creating and Using a Database You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient: name, owner, species, sex, birth, and death. Use a CREATE TABLE statement to specify the layout of your table: mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20), -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE); VARCHAR is a good choice for the name, owner, and species columns because the column values vary in length. The lengths in those column definitions need not all be the same, and need not be 20. You can normally pick any length from 1 to 65535, whatever seems most reasonable to you. If you make a poor choice and it turns out later that you need a longer field, MySQL provides an ALTER TABLE statement. Several types of values can be chosen to represent sex in animal records, such as 'm' and 'f', or perhaps 'male' and 'female'. It is simplest to use the single characters 'm' and 'f'. The use of the DATE data type for the birth and death columns is a fairly obvious choice. Once you have created a table, SHOW TABLES should produce some output: mysql> SHOW TABLES; +---------------------+ | Tables in menagerie | +---------------------+ | pet | +---------------------+ To verify that your table was created the way you expected, use a DESCRIBE statement: mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ You can use DESCRIBE any time, for example, if you forget the names of the columns in your table or what types they have. For more information about MySQL data types, see Data Types. 3.3. Loading Data into a Table After creating your table, you need to populate it. The LOAD DATA and INSERT statements are useful for this. Suppose that your pet records can be described as shown here. (Observe that MySQL expects dates in 'YYYY-MM-DD' format; this may be different from what you are used to.) name owner species sex birth death Fluffy Harold cat f 1993-02-04 Claws Gwen cat m 1994-03-17 Buffy Harold dog f 1989-05-13 Fang Benny dog m 1990-08-27 Bowser Diane dog m 1979-08-31 1995-07-29 Chirpy Gwen bird f 1998-09-11 Whistler Gwen bird 1997-12-09 Slim Benny snake m 1996-04-29 Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your an- 7
  • Creating and Using a Database imals, then load the contents of the file into the table with a single statement. You could create a text file pet.txt containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the CREATE TABLE statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use NULL values. To represent these in your text file, use N (backslash, capital-N). For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character): Whistler Gwen bird N 1997-12-09 N To load the text file pet.txt into the pet table, use this statement: mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet; If you created the file on Windows with an editor that uses rn as a line terminator, you should use this statement instead: mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet -> LINES TERMINATED BY 'rn'; (On an Apple machine running OS X, you would likely want to use LINES TERMINATED BY 'r'.) You can specify the column value separator and end of line marker explicitly in the LOAD DATA statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file pet.txt properly. If the statement fails, it is likely that your MySQL installation does not have local file capability enabled by default. See Security Issues with LOAD DATA LOCAL, for information on how to change this. When you want to add new records one at a time, the INSERT statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the CREATE TABLE statement. Suppose that Diane gets a new hamster named “Puffball.” You could add a new record using an INSERT statement like this: mysql> INSERT INTO pet -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL); String and date values are specified as quoted strings here. Also, with INSERT, you can insert NULL directly to represent a missing value. You do not use N like you do with LOAD DATA. From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several INSERT statements rather than a single LOAD DATA statement. 3.4. Retrieving Information from a Table The SELECT statement is used to pull information from a table. The general form of the statement is: SELECT what_to_select FROM which_table WHERE conditions_to_satisfy; what_to_select indicates what you want to see. This can be a list of columns, or * to indicate “all columns.” which_table in- dicates the table from which you want to retrieve data. The WHERE clause is optional. If it is present, conditions_to_satisfy specifies one or more conditions that rows must satisfy to qualify for retrieval. 3.4.1. Selecting All Data The simplest form of SELECT retrieves everything from a table: mysql> SELECT * FROM pet; +----------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+--------+---------+------+------------+------------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Fang | Benny | dog | m | 1990-08-27 | NULL | | Bowser | Diane | dog | m | 1979-08-31 | 1995-07-29 | | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | 8
  • Creating and Using a Database | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+--------+---------+------+------------+------------+ This form of SELECT is useful if you want to review your entire table, for example, after you've just loaded it with your initial data set. For example, you may happen to think that the birth date for Bowser doesn't seem quite right. Consulting your original pedigree papers, you find that the correct birth year should be 1989, not 1979. There are at least two ways to fix this: • Edit the file pet.txt to correct the error, then empty the table and reload it using DELETE and LOAD DATA: mysql> DELETE FROM pet; mysql> LOAD DATA LOCAL INFILE 'pet.txt' INTO TABLE pet; However, if you do this, you must also re-enter the record for Puffball. • Fix only the erroneous record with an UPDATE statement: mysql> UPDATE pet SET birth = '1989-08-31' WHERE name = 'Bowser'; The UPDATE changes only the record in question and does not require you to reload the table. 3.4.2. Selecting Particular Rows As shown in the preceding section, it is easy to retrieve an entire table. Just omit the WHERE clause from the SELECT statement. But typically you don't want to see the entire table, particularly when it becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer. You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this: mysql> SELECT * FROM pet WHERE name = 'Bowser'; +--------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+-------+---------+------+------------+------------+ | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+-------+---------+------+------------+------------+ The output confirms that the year is correctly recorded as 1989, not 1979. String comparisons normally are case-insensitive, so you can specify the name as 'bowser', 'BOWSER', and so forth. The query res- ult is the same. You can specify conditions on any column, not just name. For example, if you want to know which animals were born during or after 1998, test the birth column: mysql> SELECT * FROM pet WHERE birth >= '1998-1-1'; +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Puffball | Diane | hamster | f | 1999-03-30 | NULL | +----------+-------+---------+------+------------+-------+ You can combine conditions, for example, to locate female dogs: mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f'; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ The preceding query uses the AND logical operator. There is also an OR operator: mysql> SELECT * FROM pet WHERE species = 'snake' OR species = 'bird'; 9
  • Creating and Using a Database +----------+-------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+-------+ | Chirpy | Gwen | bird | f | 1998-09-11 | NULL | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | | Slim | Benny | snake | m | 1996-04-29 | NULL | +----------+-------+---------+------+------------+-------+ AND and OR may be intermixed, although AND has higher precedence than OR. If you use both operators, it is a good idea to use paren- theses to indicate explicitly how conditions should be grouped: mysql> SELECT * FROM pet WHERE (species = 'cat' AND sex = 'm') -> OR (species = 'dog' AND sex = 'f'); +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ 3.4.3. Selecting Particular Columns If you do not want to see entire rows from your table, just name the columns in which you are interested, separated by commas. For ex- ample, if you want to know when your animals were born, select the name and birth columns: mysql> SELECT name, birth FROM pet; +----------+------------+ | name | birth | +----------+------------+ | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Buffy | 1989-05-13 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Puffball | 1999-03-30 | +----------+------------+ To find out who owns pets, use this query: mysql> SELECT owner FROM pet; +--------+ | owner | +--------+ | Harold | | Gwen | | Harold | | Benny | | Diane | | Gwen | | Gwen | | Benny | | Diane | +--------+ Notice that the query simply retrieves the owner column from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword DISTINCT: mysql> SELECT DISTINCT owner FROM pet; +--------+ | owner | +--------+ | Benny | | Diane | | Gwen | | Harold | +--------+ You can use a WHERE clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query: mysql> SELECT name, species, birth FROM pet -> WHERE species = 'dog' OR species = 'cat'; +--------+---------+------------+ | name | species | birth | +--------+---------+------------+ 10
  • Creating and Using a Database | Fluffy | cat | 1993-02-04 | | Claws | cat | 1994-03-17 | | Buffy | dog | 1989-05-13 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | +--------+---------+------------+ 3.4.4. Sorting Rows You may have noticed in the preceding examples that the result rows are displayed in no particular order. It is often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an ORDER BY clause. Here are animal birthdays, sorted by date: mysql> SELECT name, birth FROM pet ORDER BY birth; +----------+------------+ | name | birth | +----------+------------+ | Buffy | 1989-05-13 | | Bowser | 1989-08-31 | | Fang | 1990-08-27 | | Fluffy | 1993-02-04 | | Claws | 1994-03-17 | | Slim | 1996-04-29 | | Whistler | 1997-12-09 | | Chirpy | 1998-09-11 | | Puffball | 1999-03-30 | +----------+------------+ On character type columns, sorting — like all other comparison operations — is normally performed in a case-insensitive fashion. This means that the order is undefined for columns that are identical except for their case. You can force a case-sensitive sort for a column by using BINARY like so: ORDER BY BINARY col_name. The default sort order is ascending, with smallest values first. To sort in reverse (descending) order, add the DESC keyword to the name of the column you are sorting by: mysql> SELECT name, birth FROM pet ORDER BY birth DESC; +----------+------------+ | name | birth | +----------+------------+ | Puffball | 1999-03-30 | | Chirpy | 1998-09-11 | | Whistler | 1997-12-09 | | Slim | 1996-04-29 | | Claws | 1994-03-17 | | Fluffy | 1993-02-04 | | Fang | 1990-08-27 | | Bowser | 1989-08-31 | | Buffy | 1989-05-13 | +----------+------------+ You can sort on multiple columns, and you can sort different columns in different directions. For example, to sort by type of animal in ascending order, then by birth date within animal type in descending order (youngest animals first), use the following query: mysql> SELECT name, species, birth FROM pet -> ORDER BY species, birth DESC; +----------+---------+------------+ | name | species | birth | +----------+---------+------------+ | Chirpy | bird | 1998-09-11 | | Whistler | bird | 1997-12-09 | | Claws | cat | 1994-03-17 | | Fluffy | cat | 1993-02-04 | | Fang | dog | 1990-08-27 | | Bowser | dog | 1989-08-31 | | Buffy | dog | 1989-05-13 | | Puffball | hamster | 1999-03-30 | | Slim | snake | 1996-04-29 | +----------+---------+------------+ The DESC keyword applies only to the column name immediately preceding it (birth); it does not affect the species column sort order. 3.4.5. Date Calculations 11
  • Creating and Using a Database MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates. To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years. mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | +----------+------------+------------+------+ Here, YEAR() pulls out the year part of a date and RIGHT() pulls off the rightmost five characters that represent the MM-DD (calendar year) part of the date. The part of the expression that compares the MM-DD values evaluates to 1 or 0, which adjusts the year difference down a year if CURDATE() occurs earlier in the year than birth. The full expression is somewhat ungainly, so an alias (age) is used to make the output column label more meaningful. The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an ORDER BY name clause to sort the output by name: mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY name; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | +----------+------------+------------+------+ To sort the output by age rather than name, just use a different ORDER BY clause: mysql> SELECT name, birth, CURDATE(), -> (YEAR(CURDATE())-YEAR(birth)) -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5)) -> AS age -> FROM pet ORDER BY age; +----------+------------+------------+------+ | name | birth | CURDATE() | age | +----------+------------+------------+------+ | Chirpy | 1998-09-11 | 2003-08-19 | 4 | | Puffball | 1999-03-30 | 2003-08-19 | 4 | | Whistler | 1997-12-09 | 2003-08-19 | 5 | | Slim | 1996-04-29 | 2003-08-19 | 7 | | Claws | 1994-03-17 | 2003-08-19 | 9 | | Fluffy | 1993-02-04 | 2003-08-19 | 10 | | Fang | 1990-08-27 | 2003-08-19 | 12 | | Bowser | 1989-08-31 | 2003-08-19 | 13 | | Buffy | 1989-05-13 | 2003-08-19 | 14 | +----------+------------+------------+------+ A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether the death value is NULL. Then, for those with non-NULL values, compute the difference between the death and birth val- ues: 12
  • Creating and Using a Database mysql> SELECT name, birth, death, -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5)) -> AS age -> FROM pet WHERE death IS NOT NULL ORDER BY age; +--------+------------+------------+------+ | name | birth | death | age | +--------+------------+------------+------+ | Bowser | 1989-08-31 | 1995-07-29 | 5 | +--------+------------+------------+------+ The query uses death IS NOT NULL rather than death <> NULL because NULL is a special value that cannot be compared us- ing the usual comparison operators. This is discussed later. See Section 3.4.6, “Working with NULL Values”. What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the birth column. MySQL provides several functions for extracting parts of dates, such as YEAR(), MONTH(), and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth): mysql> SELECT name, birth, MONTH(birth) FROM pet; +----------+------------+--------------+ | name | birth | MONTH(birth) | +----------+------------+--------------+ | Fluffy | 1993-02-04 | 2 | | Claws | 1994-03-17 | 3 | | Buffy | 1989-05-13 | 5 | | Fang | 1990-08-27 | 8 | | Bowser | 1989-08-31 | 8 | | Chirpy | 1998-09-11 | 9 | | Whistler | 1997-12-09 | 12 | | Slim | 1996-04-29 | 4 | | Puffball | 1999-03-30 | 3 | +----------+------------+--------------+ Finding animals with birthdays in the upcoming month is also simple. Suppose that the current month is April. Then the month value is 4 and you can look for animals born in May (month 5) like this: mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5; +-------+------------+ | name | birth | +-------+------------+ | Buffy | 1989-05-13 | +-------+------------+ There is a small complication if the current month is December. You cannot merely add one to the month number (12) and look for an- imals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1). You can write the query so that it works no matter what the current month is, so that you do not have to use the number for a particular month. DATE_ADD() allows you to add a time interval to a given date. If you add a month to the value of CURDATE(), then extract the month part with MONTH(), the result produces the month in which to look for birthdays: mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(),INTERVAL 1 MONTH)); A different way to accomplish the same task is to add 1 to get the next month after the current one after using the modulo function (MOD) to wrap the month value to 0 if it is currently 12: mysql> SELECT name, birth FROM pet -> WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1; MONTH() returns a number between 1 and 12. And MOD(something,12) returns a number between 0 and 11. So the addition has to be after the MOD(), otherwise we would go from November (11) to January (1). 3.4.6. Working with NULL Values The NULL value can be surprising until you get used to it. Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values. To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or <>. To demonstrate this for yourself, try the following query: mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL; +----------+-----------+----------+----------+ | 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL | 13
  • Creating and Using a Database +----------+-----------+----------+----------+ | NULL | NULL | NULL | NULL | +----------+-----------+----------+----------+ Clearly you get no meaningful results from these comparisons. Use the IS NULL and IS NOT NULL operators instead: mysql> SELECT 1 IS NULL, 1 IS NOT NULL; +-----------+---------------+ | 1 IS NULL | 1 IS NOT NULL | +-----------+---------------+ | 0 | 1 | +-----------+---------------+ In MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1. This special treatment of NULL is why, in the previous section, it was necessary to determine which animals are no longer alive using death IS NOT NULL instead of death <> NULL. Two NULL values are regarded as equal in a GROUP BY. When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC. A common error when working with NULL is to assume that it is not possible to insert a zero or an empty string into a column defined as NOT NULL, but this is not the case. These are in fact values, whereas NULL means “not having a value.” You can test this easily enough by using IS [NOT] NULL as shown: mysql> SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL; +-----------+---------------+------------+----------------+ | 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL | +-----------+---------------+------------+----------------+ | 0 | 1 | 0 | 1 | +-----------+---------------+------------+----------------+ Thus it is entirely possible to insert a zero or empty string into a NOT NULL column, as these are in fact NOT NULL. See Problems with NULL Values. 3.4.7. Pattern Matching MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as vi, grep, and sed. SQL pattern matching allows you to use “_” to match any single character and “%” to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case-insensitive by default. Some examples are shown here. You do not use = or <> when you use SQL patterns; use the LIKE or NOT LIKE comparison operators instead. To find names beginning with “b”: mysql> SELECT * FROM pet WHERE name LIKE 'b%'; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+ To find names ending with “fy”: mysql> SELECT * FROM pet WHERE name LIKE '%fy'; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+ To find names containing a “w”: mysql> SELECT * FROM pet WHERE name LIKE '%w%'; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | 14
  • Creating and Using a Database +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | +----------+-------+---------+------+------------+------------+ To find names containing exactly five characters, use five instances of the “_” pattern character: mysql> SELECT * FROM pet WHERE name LIKE '_____'; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the REGEXP and NOT REGEXP operators (or RLIKE and NOT RLIKE, which are synonyms). The following list describes some characteristics of extended regular expressions: • “.” matches any single character. • A character class “[...]” matches any character within the brackets. For example, “[abc]” matches “a”, “b”, or “c”. To name a range of characters, use a dash. “[a-z]” matches any letter, whereas “[0-9]” matches any digit. • “*” matches zero or more instances of the thing preceding it. For example, “x*” matches any number of “x” characters, “[0-9]*” matches any number of digits, and “.*” matches any number of anything. • A REGEXP pattern match succeeds if the pattern matches anywhere in the value being tested. (This differs from a LIKE pattern match, which succeeds only if the pattern matches the entire value.) • To anchor a pattern so that it must match the beginning or end of the value being tested, use “^” at the beginning or “$” at the end of the pattern. To demonstrate how extended regular expressions work, the LIKE queries shown previously are rewritten here to use REGEXP. To find names beginning with “b”, use “^” to match the beginning of the name: mysql> SELECT * FROM pet WHERE name REGEXP '^b'; +--------+--------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+------------+ | Buffy | Harold | dog | f | 1989-05-13 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | +--------+--------+---------+------+------------+------------+ If you really want to force a REGEXP comparison to be case sensitive, use the BINARY keyword to make one of the strings a binary string. This query matches only lowercase “b” at the beginning of a name: mysql> SELECT * FROM pet WHERE name REGEXP BINARY '^b'; To find names ending with “fy”, use “$” to match the end of the name: mysql> SELECT * FROM pet WHERE name REGEXP 'fy$'; +--------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +--------+--------+---------+------+------------+-------+ | Fluffy | Harold | cat | f | 1993-02-04 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +--------+--------+---------+------+------------+-------+ To find names containing a “w”, use this query: mysql> SELECT * FROM pet WHERE name REGEXP 'w'; +----------+-------+---------+------+------------+------------+ | name | owner | species | sex | birth | death | +----------+-------+---------+------+------------+------------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 | | Whistler | Gwen | bird | NULL | 1997-12-09 | NULL | 15
  • Creating and Using a Database +----------+-------+---------+------+------------+------------+ Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wild- card on either side of the pattern to get it to match the entire value like it would be if you used an SQL pattern. To find names containing exactly five characters, use “^” and “$” to match the beginning and end of the name, and five instances of “.” in between: mysql> SELECT * FROM pet WHERE name REGEXP '^.....$'; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ You could also write the previous query using the {n} (“repeat-n-times”) operator: mysql> SELECT * FROM pet WHERE name REGEXP '^.{5}$'; +-------+--------+---------+------+------------+-------+ | name | owner | species | sex | birth | death | +-------+--------+---------+------+------------+-------+ | Claws | Gwen | cat | m | 1994-03-17 | NULL | | Buffy | Harold | dog | f | 1989-05-13 | NULL | +-------+--------+---------+------+------------+-------+ Regular Expressions, provides more information about the syntax for regular expressions. 3.4.8. Counting Rows Databases are often used to answer the question, “How often does a certain type of data occur in a table?” For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of census operations on your animals. Counting the total number of animals you have is the same question as “How many rows are in the pet table?” because there is one re- cord per pet. COUNT(*) counts the number of rows, so the query to count your animals looks like this: mysql> SELECT COUNT(*) FROM pet; +----------+ | COUNT(*) | +----------+ | 9 | +----------+ Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has: mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner; +--------+----------+ | owner | COUNT(*) | +--------+----------+ | Benny | 2 | | Diane | 2 | | Gwen | 3 | | Harold | 2 | +--------+----------+ The preceding query uses GROUP BY to group all records for each owner. The use of COUNT() in conjunction with GROUP BY is useful for characterizing your data under various groupings. The following examples show different ways to perform animal census op- erations. Number of animals per species: mysql> SELECT species, COUNT(*) FROM pet GROUP BY species; +---------+----------+ | species | COUNT(*) | +---------+----------+ | bird | 2 | | cat | 2 | | dog | 3 | | hamster | 1 | | snake | 1 | +---------+----------+ 16
  • Creating and Using a Database Number of animals per sex: mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex; +------+----------+ | sex | COUNT(*) | +------+----------+ | NULL | 1 | | f | 4 | | m | 4 | +------+----------+ (In this output, NULL indicates that the sex is unknown.) Number of animals per combination of species and sex: mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | NULL | 1 | | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+ You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this: mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE species = 'dog' OR species = 'cat' -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | +---------+------+----------+ Or, if you wanted the number of animals per sex only for animals whose sex is known: mysql> SELECT species, sex, COUNT(*) FROM pet -> WHERE sex IS NOT NULL -> GROUP BY species, sex; +---------+------+----------+ | species | sex | COUNT(*) | +---------+------+----------+ | bird | f | 1 | | cat | f | 1 | | cat | m | 1 | | dog | f | 1 | | dog | m | 2 | | hamster | f | 1 | | snake | m | 1 | +---------+------+----------+ If you name columns to select in addition to the COUNT() value, a GROUP BY clause should be present that names those same columns. Otherwise, the following occurs: • If the ONLY_FULL_GROUP_BY SQL mode is enabled, an error occurs: mysql> SET sql_mode = 'ONLY_FULL_GROUP_BY'; Query OK, 0 rows affected (0.00 sec) mysql> SELECT owner, COUNT(*) FROM pet; ERROR 1140 (42000): Mixing of GROUP columns (MIN(),MAX(),COUNT()...) with no GROUP columns is illegal if there is no GROUP BY clause • If ONLY_FULL_GROUP_BY is not enabled, the query is processed by treating all rows as a single group, but the value selected for each named column is indeterminate. The server is free to select the value from any row: 17
  • Creating and Using a Database mysql> SET sql_mode = ''; Query OK, 0 rows affected (0.00 sec) mysql> SELECT owner, COUNT(*) FROM pet; +--------+----------+ | owner | COUNT(*) | +--------+----------+ | Harold | 8 | +--------+----------+ 1 row in set (0.00 sec) See also GROUP BY and HAVING with Hidden Columns. 3.4.9. Using More Than one Table The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs to contain the following in- formation: • The pet name so that you know which animal each event pertains to. • A date so that you know when the event occurred. • A field to describe the event. • An event type field, if you want to be able to categorize events. Given these considerations, the CREATE TABLE statement for the event table might look like this: mysql> CREATE TABLE event (name VARCHAR(20), date DATE, -> type VARCHAR(15), remark VARCHAR(255)); As with the pet table, it is easiest to load the initial records by creating a tab-delimited text file containing the following information. name date type remark Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male Buffy 1994-06-19 litter 3 puppies, 3 female Chirpy 1999-03-21 vet needed beak straightened Slim 1997-08-03 vet broken rib Bowser 1991-10-12 kennel Fang 1991-10-12 kennel Fang 1998-08-28 birthday Gave him a new chew toy Claws 1998-03-17 birthday Gave him a new flea collar Whistler 1998-12-09 birthday First birthday Load the records like this: mysql> LOAD DATA LOCAL INFILE 'event.txt' INTO TABLE event; Based on what you have learned from the queries that you have run on the pet table, you should be able to perform retrievals on the re- cords in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask? Suppose that you want to find out the ages at which each pet had its litters. We saw earlier how to calculate ages from two dates. The litter date of the mother is in the event table, but to calculate her age on that date you need her birth date, which is stored in the pet table. This means the query requires both tables: 18
  • Creating and Using a Database mysql> SELECT pet.name, -> (YEAR(date)-YEAR(birth)) - (RIGHT(date,5)<RIGHT(birth,5)) AS age, -> remark -> FROM pet INNER JOIN event -> ON pet.name = event.name -> WHERE event.type = 'litter'; +--------+------+-----------------------------+ | name | age | remark | +--------+------+-----------------------------+ | Fluffy | 2 | 4 kittens, 3 female, 1 male | | Buffy | 4 | 5 puppies, 2 female, 3 male | | Buffy | 5 | 3 puppies, 3 female | +--------+------+-----------------------------+ There are several things to note about this query: • The FROM clause joins two tables because the query needs to pull information from both of them. • When combining (joining) information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a name column. The query uses an ON clause to match up records in the two tables based on the name values. The query uses an INNER JOIN to combine the tables. An INNER JOIN allows for rows from either table to appear in the result if and only if both tables meet the conditions specified in the ON clause. In this example, the ON clause specifies that the name column in the pet table must match the name column in the event table. If a name appears in one table but not the other, the row will not appear in the result because the condition in the ON clause fails. • Because the name column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name. You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to produce candidate pairs of males and females of like species: mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species -> FROM pet AS p1 INNER JOIN pet AS p2 -> ON p1.species = p2.species AND p1.sex = 'f' AND p2.sex = 'm'; +--------+------+--------+------+---------+ | name | sex | name | sex | species | +--------+------+--------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | m | dog | | Buffy | f | Bowser | m | dog | +--------+------+--------+------+---------+ In this query, we specify aliases for the table name to refer to the columns and keep straight which instance of the table each column ref- erence is associated with. 19
  • Chapter 4. Getting Information About Databases and Tables What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports. You have previously seen SHOW DATABASES, which lists the databases managed by the server. To find out which database is cur- rently selected, use the DATABASE() function: mysql> SELECT DATABASE(); +------------+ | DATABASE() | +------------+ | menagerie | +------------+ If you have not yet selected any database, the result is NULL. To find out what tables the default database contains (for example, when you are not sure about the name of a table), use this command: mysql> SHOW TABLES; +---------------------+ | Tables_in_menagerie | +---------------------+ | event | | pet | +---------------------+ The name of the column in the output produced by this statement is always Tables_in_db_name, where db_name is the name of the database. See SHOW TABLES Syntax, for more information. If you want to find out about the structure of a table, the DESCRIBE command is useful; it displays information about each of a table's columns: mysql> DESCRIBE pet; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | owner | varchar(20) | YES | | NULL | | | species | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | birth | date | YES | | NULL | | | death | date | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ Field indicates the column name, Type is the data type for the column, NULL indicates whether the column can contain NULL val- ues, Key indicates whether the column is indexed, and Default specifies the column's default value. Extra displays special informa- tion about columns: If a column was created with the AUTO_INCREMENT option, the value will be auto_increment rather than empty. DESC is a short form of DESCRIBE. See DESCRIBE Syntax, for more information. You can obtain the CREATE TABLE statement necessary to create an existing table using the SHOW CREATE TABLE statement. See SHOW CREATE TABLE Syntax. If you have indexes on a table, SHOW INDEX FROM tbl_name produces information about them. See SHOW INDEX Syntax, for more about this statement. 20
  • Chapter 5. Using mysql in Batch Mode In the previous sections, you used mysql interactively to enter queries and view the results. You can also run mysql in batch mode. To do this, put the commands you want to run in a file, then tell mysql to read its input from the file: shell> mysql < batch-file If you are running mysql under Windows and have some special characters in the file that cause problems, you can do this: C:> mysql -e "source batch-file" If you need to specify connection parameters on the command line, the command might look like this: shell> mysql -h host -u user -p < batch-file Enter password: ******** When you use mysql this way, you are creating a script file, then executing the script. If you want the script to continue even if some of the statements in it produce errors, you should use the --force command-line op- tion. Why use a script? Here are a few reasons: • If you run a query repeatedly (say, every day or every week), making it a script allows you to avoid retyping it each time you ex- ecute it. • You can generate new queries from existing ones that are similar by copying and editing script files. • Batch mode can also be useful while you're developing a query, particularly for multiple-line commands or multiple-statement se- quences of commands. If you make a mistake, you don't have to retype everything. Just edit your script to correct the error, then tell mysql to execute it again. • If you have a query that produces a lot of output, you can run the output through a pager rather than watching it scroll off the top of your screen: shell> mysql < batch-file | more • You can catch the output in a file for further processing: shell> mysql < batch-file > mysql.out • You can distribute your script to other people so that they can also run the commands. • Some situations do not allow for interactive use, for example, when you run a query from a cron job. In this case, you must use batch mode. The default output format is different (more concise) when you run mysql in batch mode than when you use it interactively. For ex- ample, the output of SELECT DISTINCT species FROM pet looks like this when mysql is run interactively: +---------+ | species | +---------+ | bird | | cat | | dog | | hamster | | snake | +---------+ In batch mode, the output looks like this instead: species bird cat 21
  • Using mysql in Batch Mode dog hamster snake If you want to get the interactive output format in batch mode, use mysql -t. To echo to the output the commands that are executed, use mysql -vvv. You can also use scripts from the mysql prompt by using the source command or . command: mysql> source filename; mysql> . filename See Executing SQL Statements from a Text File, for more information. 22
  • Chapter 6. Examples of Common Queries Here are examples of how to solve some common problems with MySQL. Some of the examples use the table shop to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (article, dealer) is a primary key for the records. Start the command-line tool mysql and select a database: shell> mysql your-database-name (In most MySQL installations, you can use the database named test). You can create and populate the example table with these statements: CREATE TABLE shop ( article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL, dealer CHAR(20) DEFAULT '' NOT NULL, price DOUBLE(16,2) DEFAULT '0.00' NOT NULL, PRIMARY KEY(article, dealer)); INSERT INTO shop VALUES (1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45), (3,'C',1.69),(3,'D',1.25),(4,'D',19.95); After issuing the statements, the table should have the following contents: SELECT * FROM shop; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | A | 3.45 | | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | B | 1.45 | | 0003 | C | 1.69 | | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+ 6.1. The Maximum Value for a Column “What is the highest item number?” SELECT MAX(article) AS article FROM shop; +---------+ | article | +---------+ | 4 | +---------+ 6.2. The Row Holding the Maximum of a Certain Column Task: Find the number, dealer, and price of the most expensive article. This is easily done with a subquery: SELECT article, dealer, price FROM shop WHERE price=(SELECT MAX(price) FROM shop); +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0004 | D | 19.95 | +---------+--------+-------+ Other solutions are to use a LEFT JOIN or to sort all rows descending by price and get only the first row using the MySQL-specific LIMIT clause: SELECT s1.article, s1.dealer, s1.price FROM shop s1 23
  • Examples of Common Queries LEFT JOIN shop s2 ON s1.price < s2.price WHERE s2.article IS NULL; SELECT article, dealer, price FROM shop ORDER BY price DESC LIMIT 1; Note If there were several most expensive articles, each with a price of 19.95, the LIMIT solution would show only one of them. 6.3. Maximum of Column per Group Task: Find the highest price per article. SELECT article, MAX(price) AS price FROM shop GROUP BY article; +---------+-------+ | article | price | +---------+-------+ | 0001 | 3.99 | | 0002 | 10.99 | | 0003 | 1.69 | | 0004 | 19.95 | +---------+-------+ 6.4. The Rows Holding the Group-wise Maximum of a Certain Column Task: For each article, find the dealer or dealers with the most expensive price. This problem can be solved with a subquery like this one: SELECT article, dealer, price FROM shop s1 WHERE price=(SELECT MAX(s2.price) FROM shop s2 WHERE s1.article = s2.article); +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0001 | B | 3.99 | | 0002 | A | 10.99 | | 0003 | C | 1.69 | | 0004 | D | 19.95 | +---------+--------+-------+ The preceding example uses a correlated subquery, which can be inefficient (see Correlated Subqueries). Other possibilities for solving the problem are to use an uncorrelated subquery in the FROM clause or a LEFT JOIN: SELECT s1.article, dealer, s1.price FROM shop s1 JOIN ( SELECT article, MAX(price) AS price FROM shop GROUP BY article) AS s2 ON s1.article = s2.article AND s1.price = s2.price; SELECT s1.article, s1.dealer, s1.price FROM shop s1 LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price WHERE s2.article IS NULL; The LEFT JOIN works on the basis that when s1.price is at its maximum value, there is no s2.price with a greater value and the s2 rows values will be NULL. See JOIN Syntax. 6.5. Using User-Defined Variables You can employ MySQL user variables to remember results without having to store them in temporary variables in the client. (See User-Defined Variables.) 24
  • Examples of Common Queries For example, to find the articles with the highest and lowest price you can do this: mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop; mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price; +---------+--------+-------+ | article | dealer | price | +---------+--------+-------+ | 0003 | D | 1.25 | | 0004 | D | 19.95 | +---------+--------+-------+ Note It is also possible to store the name of a database object such as a table or a column in a user variable and then to use this variable in an SQL statement; however, this requires the use of a prepared statement. See SQL Syntax for Prepared State- ments, for more information. 6.6. Using Foreign Keys In MySQL, InnoDB tables support checking of foreign key constraints. See The InnoDB Storage Engine, and Foreign Keys. A foreign key constraint is not required merely to join two tables. For storage engines other than InnoDB, it is possible when defining a column to use a REFERENCES tbl_name(col_name) clause, which has no actual effect, and serves only as a memo or comment to you that the column which you are currently defining is intended to refer to a column in another table. It is extremely important to realize when using this syntax that: • MySQL does not perform any sort of CHECK to make sure that col_name actually exists in tbl_name (or even that tbl_name itself exists). • MySQL does not perform any sort of action on tbl_name such as deleting rows in response to actions taken on rows in the table which you are defining; in other words, this syntax induces no ON DELETE or ON UPDATE behavior whatsoever. (Although you can write an ON DELETE or ON UPDATE clause as part of the REFERENCES clause, it is also ignored.) • This syntax creates a column; it does not create any sort of index or key. You can use a column so created as a join column, as shown here: CREATE TABLE person ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, name CHAR(60) NOT NULL, PRIMARY KEY (id) ); CREATE TABLE shirt ( id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, style ENUM('t-shirt', 'polo', 'dress') NOT NULL, color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL, owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id), PRIMARY KEY (id) ); INSERT INTO person VALUES (NULL, 'Antonio Paz'); SELECT @last := LAST_INSERT_ID(); INSERT INTO shirt VALUES (NULL, 'polo', 'blue', @last), (NULL, 'dress', 'white', @last), (NULL, 't-shirt', 'blue', @last); INSERT INTO person VALUES (NULL, 'Lilliana Angelovska'); SELECT @last := LAST_INSERT_ID(); INSERT INTO shirt VALUES (NULL, 'dress', 'orange', @last), (NULL, 'polo', 'red', @last), (NULL, 'dress', 'blue', @last), (NULL, 't-shirt', 'white', @last); SELECT * FROM person; +----+---------------------+ | id | name | +----+---------------------+ | 1 | Antonio Paz | | 2 | Lilliana Angelovska | +----+---------------------+ SELECT * FROM shirt; +----+---------+--------+-------+ | id | style | color | owner | +----+---------+--------+-------+ | 1 | polo | blue | 1 | | 2 | dress | white | 1 | 25
  • Examples of Common Queries | 3 | t-shirt | blue | 1 | | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | | 7 | t-shirt | white | 2 | +----+---------+--------+-------+ SELECT s.* FROM person p INNER JOIN shirt s ON s.owner = p.id WHERE p.name LIKE 'Lilliana%' AND s.color <> 'white'; +----+-------+--------+-------+ | id | style | color | owner | +----+-------+--------+-------+ | 4 | dress | orange | 2 | | 5 | polo | red | 2 | | 6 | dress | blue | 2 | +----+-------+--------+-------+ When used in this fashion, the REFERENCES clause is not displayed in the output of SHOW CREATE TABLE or DESCRIBE: SHOW CREATE TABLE shirtG *************************** 1. row *************************** Table: shirt Create Table: CREATE TABLE `shirt` ( `id` smallint(5) unsigned NOT NULL auto_increment, `style` enum('t-shirt','polo','dress') NOT NULL, `color` enum('red','blue','orange','white','black') NOT NULL, `owner` smallint(5) unsigned NOT NULL, PRIMARY KEY (`id`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 The use of REFERENCES in this way as a comment or “reminder” in a column definition works with MyISAM tables. 6.7. Searching on Two Keys An OR using a single key is well optimized, as is the handling of AND. The one tricky case is that of searching on two different keys combined with OR: SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1' OR field2_index = '1' This case is optimized. See Index Merge Optimization. You can also solve the problem efficiently by using a UNION that combines the output of two separate SELECT statements. See UNION Syntax. Each SELECT searches only one key and can be optimized: SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1' UNION SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1'; 6.8. Calculating Visits Per Day The following example shows how you can use the bit group functions to calculate the number of days per month a user has visited a Web page. CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL, day INT(2) UNSIGNED ZEROFILL); INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2), (2000,2,23),(2000,2,23); The example table contains year-month-day values representing visits by users to the page. To determine how many different days in each month these visits occur, use this query: SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month; 26
  • Examples of Common Queries Which returns: +------+-------+------+ | year | month | days | +------+-------+------+ | 2000 | 01 | 3 | | 2000 | 02 | 2 | +------+-------+------+ The query calculates how many different days appear in the table for each year/month combination, with automatic removal of duplicate entries. 6.9. Using AUTO_INCREMENT The AUTO_INCREMENT attribute can be used to generate a unique identity for new rows: CREATE TABLE animals ( id MEDIUMINT NOT NULL AUTO_INCREMENT, name CHAR(30) NOT NULL, PRIMARY KEY (id) ); INSERT INTO animals (name) VALUES ('dog'),('cat'),('penguin'), ('lax'),('whale'),('ostrich'); SELECT * FROM animals; Which returns: +----+---------+ | id | name | +----+---------+ | 1 | dog | | 2 | cat | | 3 | penguin | | 4 | lax | | 5 | whale | | 6 | ostrich | +----+---------+ You can retrieve the most recent AUTO_INCREMENT value with the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. These functions are connection-specific, so their return values are not affected by another connection which is also performing inserts. Note For a multiple-row insert, LAST_INSERT_ID() and mysql_insert_id() actually return the AUTO_INCREMENT key from the first of the inserted rows. This allows multiple-row inserts to be reproduced correctly on other servers in a replication setup. For MyISAM tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given- prefix. This is useful when you want to put data into ordered groups. CREATE TABLE animals ( grp ENUM('fish','mammal','bird') NOT NULL, id MEDIUMINT NOT NULL AUTO_INCREMENT, name CHAR(30) NOT NULL, PRIMARY KEY (grp,id) ) ENGINE=MyISAM; INSERT INTO animals (grp,name) VALUES ('mammal','dog'),('mammal','cat'), ('bird','penguin'),('fish','lax'),('mammal','whale'), ('bird','ostrich'); SELECT * FROM animals ORDER BY grp,id; Which returns: +--------+----+---------+ | grp | id | name | +--------+----+---------+ | fish | 1 | lax | | mammal | 1 | dog | | mammal | 2 | cat | | mammal | 3 | whale | 27
  • Examples of Common Queries | bird | 1 | penguin | | bird | 2 | ostrich | +--------+----+---------+ In this case (when the AUTO_INCREMENT column is part of a multiple-column index), AUTO_INCREMENT values are reused if you delete the row with the biggest AUTO_INCREMENT value in any group. This happens even for MyISAM tables, for which AUTO_INCREMENT values normally are not reused. If the AUTO_INCREMENT column is part of multiple indexes, MySQL will generate sequence values using the index that begins with the AUTO_INCREMENT column, if there is one. For example, if the animals table contained indexes PRIMARY KEY (grp, id) and INDEX (id), MySQL would ignore the PRIMARY KEY for generating sequence values. As a result, the table would contain a single sequence, not a sequence per grp value. To start with an AUTO_INCREMENT value other than 1, you can set that value with CREATE TABLE or ALTER TABLE, like this: mysql> ALTER TABLE tbl AUTO_INCREMENT = 100; More information about AUTO_INCREMENT is available here: • How to assign the AUTO_INCREMENT attribute to a column: CREATE TABLE Syntax, and ALTER TABLE Syntax. • How AUTO_INCREMENT behaves depending on the SQL mode: Server SQL Modes. • How to find the row that contains the most recent AUTO_INCREMENT value: Comparison Functions and Operators. • Setting the AUTO_INCREMENT value to be used: Session System Variables. • AUTO_INCREMENT and replication: Replication Features and Issues. • Server-system variables related to AUTO_INCREMENT (auto_increment_increment and auto_increment_offset) that can be used for replication: Server System Variables. 28
  • Chapter 7. Queries from the Twin Project At the places the early MySQL was developed (Analytikerna and Lentus), the founders did systems and field work for a big research project. This project was a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California. The project involved lots of data collection from all twins in Sweden older than 65 Years (see ht- tp://ki.se/ki/jsp/polopoly.jsp?d=9610&l=en). Large parts of the project were administered with a Web interface written using Perl and MySQL. 7.1. Find All Nondistributed Twins The following query was used to determine what twins should be studied further after a initial screening. This was written around 1997 using MySQL 3.19. SELECT CONCAT(p1.id, p1.tvab) + 0 AS tvid, CONCAT(p1.christian_name, ' ', p1.surname) AS Name, p1.postal_code AS Code, p1.city AS City, pg.abrev AS Area, IF(td.participation = 'Aborted', 'A', ' ') AS A, p1.dead AS dead1, l.event AS event1, td.suspect AS tsuspect1, id.suspect AS isuspect1, td.severe AS tsevere1, id.severe AS isevere1, p2.dead AS dead2, l2.event AS event2, h2.nurse AS nurse2, h2.doctor AS doctor2, td2.suspect AS tsuspect2, id2.suspect AS isuspect2, td2.severe AS tsevere2, id2.severe AS isevere2, l.finish_date FROM twin_project AS tp /* For Twin 1 */ LEFT JOIN twin_data AS td ON tp.id = td.id AND tp.tvab = td.tvab LEFT JOIN informant_data AS id ON tp.id = id.id AND tp.tvab = id.tvab LEFT JOIN harmony AS h ON tp.id = h.id AND tp.tvab = h.tvab LEFT JOIN lentus AS l ON tp.id = l.id AND tp.tvab = l.tvab /* For Twin 2 */ LEFT JOIN twin_data AS td2 ON p2.id = td2.id AND p2.tvab = td2.tvab LEFT JOIN informant_data AS id2 ON p2.id = id2.id AND p2.tvab = id2.tvab LEFT JOIN harmony AS h2 ON p2.id = h2.id AND p2.tvab = h2.tvab LEFT JOIN lentus AS l2 ON p2.id = l2.id AND p2.tvab = l2.tvab, person_data AS p1, person_data AS p2, postal_groups AS pg WHERE /* p1 gets main twin and p2 gets his/her twin. */ /* ptvab is a field inverted from tvab */ p1.id = tp.id AND p1.tvab = tp.tvab AND p2.id = p1.id AND p2.ptvab = p1.tvab AND /* Just the screening survey */ tp.survey_no = 5 AND /* Skip if partner died before 65 but allow emigration (dead=9) */ (p2.dead = 0 OR p2.dead = 9 OR (p2.dead = 1 AND (p2.death_date = 0 OR (((TO_DAYS(p2.death_date) - TO_DAYS(p2.birthday)) / 365) >= 65)))) AND ( /* Twin is suspect */ (td.future_contact = 'Yes' AND td.suspect = 2) OR /* Twin is suspect - Informant is Blessed */ (td.future_contact = 'Yes' AND td.suspect = 1 AND id.suspect = 1) OR 29
  • Queries from the Twin Project /* No twin - Informant is Blessed */ (ISNULL(td.suspect) AND id.suspect = 1 AND id.future_contact = 'Yes') OR /* Twin broken off - Informant is Blessed */ (td.participation = 'Aborted' AND id.suspect = 1 AND id.future_contact = 'Yes') OR /* Twin broken off - No inform - Have partner */ (td.participation = 'Aborted' AND ISNULL(id.suspect) AND p2.dead = 0)) AND l.event = 'Finished' /* Get at area code */ AND SUBSTRING(p1.postal_code, 1, 2) = pg.code /* Not already distributed */ AND (h.nurse IS NULL OR h.nurse=00 OR h.doctor=00) /* Has not refused or been aborted */ AND NOT (h.status = 'Refused' OR h.status = 'Aborted' OR h.status = 'Died' OR h.status = 'Other') ORDER BY tvid; Some explanations: • CONCAT(p1.id, p1.tvab) + 0 AS tvid We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number. • Column id This identifies a pair of twins. It is an index in all tables. • Column tvab This identifies a twin in a pair. It has a value of 1 or 2. • Column ptvab This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query. This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the ex- ample, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned. All of the above exist in all tables with twin-related information. We have an index on both id, tvab (all tables), and id, ptvab (person_data) to make queries faster. When we did this work, our production machine was a 200MHz UltraSPARC, and on that old hardware this query returned about 150–200 rows in less than one second. The main table had 70k Rows. 7.2. Show a Table of Twin Pair Status Each twin has a status code called event. The query shown here is used to select all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on. SELECT t1.event, t2.event, COUNT(*) FROM lentus AS t1, lentus AS t2, twin_project AS tp WHERE /* We are looking at one pair at a time */ t1.id = tp.id AND t1.tvab=tp.tvab AND t1.id = t2.id /* Just the screening survey */ AND tp.survey_no = 5 /* This makes each pair only appear once */ AND t1.tvab='1' AND t2.tvab='2' GROUP BY 30
  • Queries from the Twin Project t1.event, t2.event; 31
  • Chapter 8. Using MySQL with Apache There are programs that let you authenticate your users from a MySQL database and also let you write your log files into a MySQL ta- ble. You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file: LogFormat ""%h",%{%Y%m%d%H%M%S}t,%>s,"%b","%{Content-Type}o", "%U","%{Referer}i","%{User-Agent}i"" To load a log file in that format into MySQL, you can use a statement something like this: LOAD DATA INFILE '/local/access_log' INTO TABLE tbl_name FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '' The named table should be created to have columns that correspond to those that the LogFormat line writes to the log file. 32